A cross-sectional view of a prior art semiconductor laser producing light visible to the naked eye is shown in FIG. 9. The laser includes an n-type gallium arsenide (GaAs) substrate 1, an n-type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 2 disposed on the GaAs substrate 1, and an undoped Ga.sub.0.5 In.sub.0.5 P active layer 3 disposed on the cladding layer 2. A p-type (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 4 is disposed on the active layer 3 and also includes a centrally disposed mesa or ridge portion. A p-type Ga.sub.0.5 In.sub.0.5 P transition layer 5 is disposed on the mesa portion of the cladding layer 4. A p-type GaAs contacting layer portion 7a is disposed on the transition layer 5 in the mesa portion. Together, the contacting layer portion 7a, the transition layer 5, and the mesa portion of the cladding layer 4 comprise a mesa 10. At both sides of the mesa 10 and on the parts of the cladding layer 4 beyond the mesa 10, an n-type GaAs current blocking layer 6 is disposed. The current blocking layer 6, of opposite conductivity type from the cladding layer 4, concentrates the current flow through the semiconductor laser at the mesa 10. A current concentration and collection structure that concentrates current flow in the central portion of the active layer 3 comprises layers 4, 5, and 7a, and the current blocking layer 6. A p-type GaAs contacting layer portion 7b is disposed on the mesa as well as on the current blocking layer 6. Electrodes 8 and 9 are disposed on the substrate 1 and the contacting layer portion 7b, respectively. The transition layer 5 reduces the abruptness of the energy gap transition between the cladding layer 4 and the contacting layer portion 7a and reduces the voltage drop that could occur at a junction directly between the cladding layer 4 and the contacting layer portion 7a.
Typically, the thicknesses of the respective layers in the structure of FIG. 9 are as follows:
TABLE 1 ______________________________________ Layer Thickness ______________________________________ cladding layer 2 1 micron active layer 3 0.08 micron cladding layer 4 0.3 micron (outside mesa 10) cladding layer 4 1 micron (within mesa 10) transition layer 5 0.1 micron current blocking layer 6 1.1 microns contacting layer portion 7a 0.3 micron contacting layer portion 7b 3 microns. ______________________________________
The transverse width of mesa 10 at its intersection with the wider part of the cladding layer 4 is about 4 microns.
In accordance with well known semiconductor laser physics, when a current flows between electrodes 8 and 9, charge carriers recombine within the active layer 3 and produce light. When the current density exceeds a certain threshold, the light produced is coherent, i.e., in phase, and laser oscillation occurs. In order to produce laser oscillation, the transverse oscillating mode, i.e., the mode along the active layer 3 dimension in the plane of FIG. 9, must be stabilized. Transverse mode stabilization is achieved in the structure of FIG. 9 by a combination of the thickness of the cladding layer 4 outside the mesa 10 and the presence of the GaAs blocking layer 6. Light produced in the active layer 3 penetrates about one-half micron into the cladding layers 2 and 4. Since the cladding layer 4 is only about 0.3 micron thick outside the mesa 10, some of the light produced in the active layer 3 penetrates into the current blocking layer 6. Since the GaAs current blocking layer has a smaller energy band gap than the active layer 3, the light is absorbed in the current blocking layer 6. On the other hand, at the mesa 10, the cladding layer 4 has a thickness of about 1 micron so that the light is not absorbed in the mesa 10, thereby stabilizing transverse mode oscillations. Effectively, the refractive index of the structure is different within and outside the mesa 10, confining light in the transverse direction in a so-called loss guide structure.
A method of making the laser structure of FIG. 9 is shown in FIGS. 10(a)-10(f). Initially, as shown in FIG. 10(a), the cladding layer 2, the active layer 3, the cladding layer 4, the transition layer 5, and the contacting layer portion 7a are successively grown on the substrate 1 in a first epitaxial growth sequence. Preferably, the layers are sequentially grown by metal organic chemical vapor deposition (MOCVD) at temperatures of about 650.degree. C. to 700.degree. C. Before the cladding layer 2 is deposited, the temperature of the GaAs substrate 1 is raised to the growth temperature. In order to avoid thermal decomposition of the substrate, an excess pressure of arsenic is provided by flowing arsine (AsH.sub.3) over the substrate. When the growth temperature is reached, for deposition of the (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 2 is begun by terminating the flow of AsH.sub.3 and introducing the source gases for growing the cladding layer 2. Typically, those gases are phosphine (PH.sub.3), trimethylaluminum, trimethylgallium, and trimethylindium.
After the epitaxial growth of the layers 2-7a, a silicon nitride (SiN) film is deposited on the contacting layer portion 7a and formed into a mask 11, as shown in FIG. 10(b). The contacting layer portion 7a, the transition layer 5, and the cladding layer 4 beyond of the mask 11 are etched to form the mesa 10. In the structure shown, the mesa 10 is a reverse mesa, i.e., the side walls converge in the direction of the active layer 3. As well known in the art, the provision of a reverse mesa, or a forward mesa in which the walls diverge in the direction of the active layer, can be chosen based upon the orientation of the mask 11 with respect to the crystalline orientation of the substrate 1. The mesa 10 produced by the etching step is shown in FIG. 10(c).
The GaAs current blocking layer 6 is grown, preferably by MOCVD, with the mask 11 in place as shown in FIG. 10(d) Since MOCVD crystal growth is selective, little GaAs deposits on the mask 11. The growth of the current blocking layer 6 comprises a second epitaxial growth sequence in which the temperature of the substrate 1 and the other previously grown layers must be raised from room temperature to the growth temperature and reduced again to room temperature after the deposition of the blocking layer 6. As the temperature is increased, before the current blocking layer 6 is grown, PH.sub.3 is passed over the structure and provides an excess pressure of phosphorus to avoid decomposition of, i.e., phosphorus loss from, the exposed (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P cladding layer 4. When the growth temperature is reached, the phosphorus supply is terminated and the source gases for growing the n-type GaAs current blocking layer 6, namely, trimethylgallium and AsH.sub.3, along with a dopant source gas such as silane (SiH.sub.4) or hydrogen selenide (H.sub.2 Se), are introduced and flow over the structure. After the current blocking layer 6 is grown, the substrate is again cooled so that the SiN mask 11 can be removed.
In a third and final epitaxial growth sequence, illustrated in FIG. 10(e), the GaAs contacting layer portion 7b is epitaxially grown on the GaAs contacting layer portion 7b and the current blocking layer 6. The electrodes 8 and 9, shown in FIG. 10(f), are deposited by conventional metal evaporation steps.
Although steps are taken as described above to avoid arsenic loss from the GaAs substrate 1 before the growth of the cladding layer 2 and to prevent phosphorus loss from the cladding layer 4 at the beginning of the second epitaxial growth sequence, it is impossible to prevent arsenic and phosphorus loss since the gaseous sources of those elements must be terminated at the growth temperatures before the source gases for the growth of additional layers are supplied. During the period at the beginning of the second epitaxial growth sequence when no phosphorus containing gas is supplied, phosphorus is lost from the cladding layer 4, producing vacancies and crystalline defects in the cladding layer 4. Since the cladding layer 4 is only about 0.3 micron thick beyond the mesa 10 and adjacent the active layer 3, phosphorus loss defects can even propagate through the cladding layer 4 to the active layer 3. The resulting vacancies act as recombination centers for charge carriers, reducing the light output of the laser and shortening its lifetime. The present invention is directed to preventing the defects, thereby increasing the light output of the laser and extending its lifetime.
One possible solution to the phosphorus loss problem is to continue the flow of PH.sub.3 during the growth of the GaAs current blocking layer 6. However, instead of growing a GaAs current blocking layer, in that event a GaAs.sub.1-x P.sub.x current blocking layer is grown. It is difficult or impossible to produce a single crystal current blocking layer under these conditions because the lattice constant GaAs.sub.1-x P.sub.x does not match the lattice constant of the cladding layer 4.
Alternatively, the current blocking layer could be (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P (0&lt;.times..ltoreq.1) instead of GaAs. If that material is employed, the phosphorus pressure can be maintained during growth of the current blocking layer 6, protecting the cladding layer 4. However, the energy band gap of (Al.sub.0.7 Ga.sub.0.3).sub.0.5 In.sub.0.5 P may be larger than the energy band gap of the Ga.sub.0.5 In.sub.0.5 P active layer 3. In that event, the light produced in the active layer 3 is not absorbed in the current blocking layer 6 outside the mesa 10 so that no transverse oscillation mode stability can be achieved.
Transverse light confinement can be achieved by other structures besides the loss guide structure of FIG. 9. For example, when the current blocking layer 6 is (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P, the aluminum composition of the cladding layer can be increased, for example, to x&gt;0.7, so that an actual rather than an effective transverse refractive index distribution confines light generated in the active layer 3 to the mesa 10. However, when the aluminum content of the current blocking layer becomes too high, it is difficult to grow (Al.sub.x Ga.sub.1-x).sub.0.5 In.sub.0.5 P epitaxially on the cladding layer 4.